The Ecology of coastal Marshes of Western Lake Erie: A Community ...

the northward plunging end of the FindlayArch (Cincinnati Antic1 ine), or A1 gonquinArch as it is known in Ontario. Glacialerosion had relative1 y st ight effects onthese resistant rocks other than to formimpressive grooves such as those found onKel leys Island and the Bass Islands. Theglacial scour was probably controlled bythe preglacial stream valleys8 resultingin the shallow basin and the islandchains.The bedrock in the island region ofwestern Lake Erie (Figure 23) issedimentary in origin and was deposited aslime muds in shallow, warm Silurian andDevonian seas, which covered the reg ionfrom 410 to 375 million years (m.y.1 ago.The warm, clear conditions of those seasc rn be inferred from the abundant fossilcr~rals and other invertebrates found inthe rocks on Kelleys and Johnson islands.While a shallow sea occupied the presentislands region, the Appalachian Mountainswere built to the east, resulting in thedeposition of deltas which now form thesandstones and shales of the NiagaraEscarpment at the east end of the Lake.The shores of all of the majorislands are rockbound, chiefly rugged incharacter, with bluffs along the majorportions of their perimeters. The highestelevations are normal 1 y adjacent to thewest. shores, except West Sister Islandwhere the bluffs are highest along theeast shore. The upland area adjacent tothe west shore of South Bass Island(Victory Woods) reaches a height of 21 mabove lake level, the highest elevation inthe islands. Small sandr cobble, orboulder beaches are situated atindentations in the shore1 ine. The mostextensive sand beach of this type is foundin the north bay of Kelleys Island.The dominant structural feature ofthe bedrock underlying western Lake Erieis the Ffndlay Arch. The nearly northsouthaxis of this arch passes through theisland region and then plunges gently tothe north. A study of the structure ofPrecambrian, or basement, rock of Ohioindicates that the crest of the arch 1 iesa few miles east of West Sister Island.As a consequence of the alignment of thearch, the over1 y ing Paleozoic bedrock dipsto the east at approximately 4 to 7 m perkm in the Bass-Kelleys Island area (Figure24). For this reason, the 01 dest rocksare exposed on West Sister Island andFigure 23. Bedrock geology of Ottawa County, Ohio (adapted from Forsyth 1971).26